Tanks for the storage of liquids have been constructed in a variety of ways from a variety of materials. In one common application, the storage of hydrocarbons, such as gasoline and other petroleum products, the tanks have conventionally been fabricated out of steel or fiberglass, most commonly with a single rigid wall. In many applications, this construction has proved reasonably satisfactory, with such tanks functioning properly for many years before requiring repair or replacement. However, the increasing age of many of the tanks currently in place is beginning to present serious environmental dangers. Many of the older steel tanks buried underground have rusted and are beginning to leak, thus releasing the petroleum materials into the ground where they may seep into and pollute underground water supplies. While rust-proof, some fiberglass tanks have also exhibited leakage, causing similar problems.
One of the primary problems with leaking storage tanks has been the difficulty or inability to ascertain when or if such leaks are occurring from a given tank. Because the excavation and removal of such a storage tank, which may contain thousands of gallons of fuel, is an expensive and difficult undertaking, such an operation is difficult to justify unless there is some evidence of actual leakage.
Because of the increasing potential danger of leaking storage tanks, particularly in communities that utilize ground water for public consumption, many municipalities have implemented or plan to implement ordinances requiring the use of double wall storage tanks underground and requiring replacement of existing single wall tanks. While the installation of a conventional double wall tank in a new facility entails no great difficulty and a generally manageable increase in cost over a single wall tank, a heavy burden exists for complying with such ordinances by replacing existing sound, single wall tanks with double wall tanks. This burden has prompted the search for methods of fabricating relatively inexpensive double wall or multi-wall tanks. This burden has also given impetus to the search for a method of remanufacturing existing single wall tanks into double or multi-wall assemblies with means for detecting the presence of any leaks into the space between the walls.
Several methods for manufacturing double wall tank assemblies have been developed, including that disclosed by David T. Palazzo, the inventor of the present invention, in U.S. Pat. No. 4,640,439 and its progeny. This patent generally discloses applying a spacing material over a rigid inner tank and bonding resin impregnated glass fiber mats to the exterior surface of spacing material to form a double wall tank. While conventional glass fiber provides a relatively effective and inexpensive outer sheath, other materials, such as polyolefins, exist that may provide a convenient alternative to forming a substantially liquid impervious outer shell around the rigid inner tank.
A thermoplastic synthetic resin material, such as polyethylene, has been extruded onto the surfaces of steel pipes as disclosed in U.S. Pat. No. 4,478,200 to Hakert, et al., and U.S. Pat. No. 3,616,006 to Landgraf, et al. However, these materials ordinarily will not bond adequately to metallic or steel surfaces, with or without adhesives, as is taught by these and other references. When fabricating a multiple wall storage tank, it is often desired, if not essential, to bond an outer jacket to the inner tank, which is typically formed of steel, around the fittings or apertures that may be formed through many conventional single wall tanks in order to prevent leakage around these fittings.